Such a turbine casing is described in an article entitled, "Steam Turbines in Conventional Combined District Heating and Power Stations," by Helmut Muhlhauser, in Brown Boveri Review No. 9, 1977, pp. 536-549 (see especially FIGS. 12g, 21, 22).
In the prior art turbine, the IP casing is non-symmetrical, with double flow, and the steam path is divided into two streams on entering the IP casing. Each outflow from this casing is at a different pressure. The two partial steam flows remain independent of one another until reaching the condenser, which is connected to the exhaust of the LP section.
Steam distribution between the LP section supply and the heat exchanger supplies is automatically controlled by means of butterfly valves installed in the exhausts feeding the LP section and control bypasses arranged in parallel in the exchangers' water system, so that both exchangers provide the same amount of power and that the water having passed through the two exchangers has the desired temperature.
Indeed, it is important for purposes of efficiency that each exchanger provide approximately the same heating power to the district heating water. The efficiency obtained with the prior art turbine is good. However, even a small further improvement in efficiency leads to a substantial energy saving when the entire life of the machine is considered.